[Elecraft] [KX1,K1,KX3] QRP Portable Antenna

Thu Aug 23 19:08:56 EDT 2012

I'm not an antenna designer or RF engineer; I'm just a reasonably average
amateur. But I got surprisingly good results with an off-center-fed antenna
made from 24-ga solid-conductor speaker wire. I connected the conductors to
a BNC-to-binder-post adapter right to the antenna output from my KX1 (with
the KXAT1). I calculated a half-wavelength at 14.060MHz for the radiator,
which works out to just over 33 feet, which got tossed up over a couple of
convenient tree branches, and half that for an elevated counterpoise that
was laid on top of some nearby shrubbery. My first contact, on battery
power, was 1800 miles. Those were extraordinary propagation conditions, to
be sure...my next four contacts with that setup averaged 800-900 miles.

My newly-acquired Buddipole hasn't had a great deal of exercise yet, but I
find similar results with it, and it has the advantages of being more
versatile and more rugged. If I rig an "L" configuration it's a bit more
directional, but the noise floor drops by an S unit or two. The only
downside to the Buddipole is that I have to retune it if I switch bands. My
OCF wire could be tuned internally to around 1.6:1 on 20m, 30m and 40m all
without moving from my lawn chair. (I don't have 80m in my radio.)

Just my 2p worth.
--
73 de N5ILN
Alan
NAQCC # 6139

On Thu, Aug 23, 2012 at 3:53 PM, Ron D'Eau Claire <ron at cobi.biz> wrote:

> The other way to increase efficiency of an end-fed wire is to reduce the
> current that needs to flow in the ground return. That's done by raising the
> impedance of the radiator at the feed point, the maximum being when it's
> 1/2
> wavelength long. Most ATU's can't handle the impedance of an exactly 1/2
> wave radiator, but the closer you can get to it, the less current flows
> into
> the radiator and so the less current flows into the "ground", ergo, lower
> ground losses due to the ground resistance.
>
> All sorts of intermediate lengths can provide good results, especially
> compared to a shortened "loaded" antenna, as long as you stay well above
> 1/4
> wavelength in radiator length.
>
> The major purpose of a "ground" return then becomes keeping the RF
> potential
> at the rig low enough to avoid problems with "RF in the shack" since the
> tuner and rig will tend to "float" up to the RF potential at the feed point
> of the radiator.
>
> 73, Ron AC7AC
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of FredJensen
> Sent: Thursday, August 23, 2012 1:45 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] [KX1,K1,KX3] QRP Portable Antenna
>
> On 8/23/2012 6:51 PM, Phil Hystad wrote:
> > I have operated my KX1 out in various parks and fields by hanging one end
> of a wire up in a tree that I would raise using fishing line and a sling
> shot.
> My usual method too.  I use the internal KXAT1 tuner, works fine.
> > A while back, Wayne said that he would use just a single wire up in the
> tree and then another wire laying on the ground as sort of a ground plane.
> My version of Wayne's antenna is a length of RG-58.  I pulled off all but a
> few inches of the outer jacket, spread the shield wires, and pulled the
> center conductor through to yield a ~26 ft antenna conductor [center
> conductor and dielectric], and a ~26 ft length of uninsulated "wire" [the
> shield braid].  A male BNC goes on the end where the coax is still intact,
> and plugs directly into the KX1.  I run the shield wire out on the ground.
>
> To radiate, you have to have RF current flowing in your antenna conductor.
> Maxwellians will tell you it involves both the "conduction"
> and "displacement" current.  The displacement current effectively
> represents
> your radiated signal.  In order to have a current, you need a closed
> circuit
> so the conduction current has a path back to the TX.
> Resistance in that path lowers the current [both conduction and
> displacement
> and hence your signal] just as Ohm's Law would suggest.
> The path can be through the earth [higher resistance], or you can provide a
> lower resistance path with a conductor(s) ... your choice.
> > 1.  Is more then a single ground wire (wire laying along the ground) good
> enough or would two or three be better?
> EZNEC4 "suggests" that one is around 85% as effective as a perfect ground.
> I say "suggests" because EZNEC4 won't model wires on the ground so I had to
> elevate it about 1/2 inch, at which point, its length becomes a minor
> confusion factor.  Two seems to gain you another 5% or so, and the benefit
> per additional wire decreases while the complexity of the system increases
> rapidly.  As you add wires on the ground, the probability that a little
> human or dog passing by your position will trip on your wires becomes 1.0
> fairly quickly.  I've compromised efficiency vs complexity to one wire [see
> above].
> > 2.  My wire antenna has usually been on the order of 50 feet but it also
> depends on how close I get for reasonable operating position (e.g. picnic
> table)?  So, would a ground wire need to be the same length or would half
> the length be as reasonable.
> The antenna length needs to be such that the impedance at the TX end can be
> matched by the tuner on the band(s) you want to operate.  32 ft is a
> 1/4 wave on 40 and will work well, it has a fairly low impedance [30-50
> ohms].  At 20m, it's a half-wave with a high impedance and may be a hard
> match for the tuner.  My KX1 does 40, 30, and 20 [I figure life is too
> short
> for 3W on 80].  The KXAT1 seems pretty happy on all three bands with my ~26
> ft length, which is sort of non-resonant on my 3 bands.
>
> On the ground, length is not critical, more is generally better, but you're
> in the field.  Do what mechanically works for you.  Mine made from RG-58
> winds onto two little crank reels a friend gave me.
> > 3.  How does this compare to a Buddipole antenna?
> I just sold my Deluxe BP because of weight.  BP's can be set up in multiple
> configurations, comparison is very difficult.  On any band but possibly
> 10m,
> a BP is a loaded antenna in any config and loading introduces losses
> however.  It is not "hard" to set up, but, so long as you have some sort of
> "organic towers" for your wire, it is quite a bit more work than tossing
> the
> line over a branch.
>
> 73,
>
> Fred K6DGW
>
>
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